[Johnson] T/R relay

Thu Jul 11 21:27:13 EDT 2013

** Yes Ed, it does make sense and the concept is far from new. When National 
switched over to a SS PS for the NCX-3 a different transformer was designed. 
It had a low impedance primary and secondary and not high impedance as used 
with tube rectifiers.OTOH receivers built by them in the 60's still had tube 
rectifiers and they didnt switch between production runs.

High vacuum rectifiers offer a soft start thanks to the transformer and a 
controlled warmup.
I very seldom recommend switching to SS, except for MV tubes, as the long 
term reliability is now controlled by just the filter caps. In the past it 
was filters plus paper cap leakage that put tube rectifiers over the edge.

Carl
KM1H

Actually, the losses=heat argument makes excellent sense. Common sense 
even!!!

I  care more about getting rid of the selenium than anything else.

** The Johnson TR does not have a selenium but I do concur about removing
them in other equipment. There it is usually required to add a series heat
generating resistor to bring the DC down to a safe level.

It DOES
take a minor load off the transformer's net power load when you replace a
tube rectifier with silicon, hard switching notwithstanding.

** Nope it does not, and here is an excellent explanation.
http://www.w8ji.com/power_transformer_stress.htm

In addition removing the 6X4 filament load would raise the resultant voltage
to the 6BL7 and with todays high AC line voltage may put it out of spec and
reduce tube life rather fast.

The DXE price includes the other hardware. I just happened to pick on them
because I knew what their Amphenol SO-239 connectors' insulation was. Mouser
DOES list it as well, and represents a better deal for sure-if you have your
own screws, etc.

** I would hope that anyone working on vintage gear has ample small parts to
work on them and doesnt have to pay more than twice what Mouser charges.
Besides you can get the 2 filter caps and other parts at the same time. I
also suggest checking some of the carbon composition resistance values as
years of RF can change them dramatically and cause problems.
$DXE$  is fine for those with few skills and a fat credit card balance;
usually no expense spared contesters, and DXers.

I do not expect T/R Switches to really improve receiver specs-I just don't
want them to hurt them. It's the nearly foolproof T/R switching that make me
a fan.

Ed Tanton

** Id be concerned about using one with SS gear; I know my NC-300, 75A4, and
other vintage gear can handle the RF for a few milliseconds.

Carl
KM1H

The only 'normal' reason for desensing with an electronic T/R Switch would
be operation on a band the TX is not tuned to. If that isn't what you're
doing, I would make certain your Fast/Slow AGC setting on the receiver isn't
the cause, then I would swap the 6BL7. After that, you start getting into
some component problem within the 250-39. One thing you can do about the
heat: paint the cabinet black. Most of whatever that finish is on my 250-39s
is crummy, one way or another (usually the front panel on mine.) So painting
the cabinet isn't going to hurt the value. This provides a nice opportunity
to 1st drill out the rivets and swap the connectors. If you care about
matching such things, DX-Engineering sells the Amphenol SO-239s with that
glass-reinforced PBT insulation material (as opposed to 'modern' TFE.) See:
http://www.dxengineering.com/parts/dxe-83-1r-kit

** More of the usual overpricing from DXE, I make it a point to buy nothing
from them.
That connector is $3.28 at Mouser who has no minimum, and low shipping when
USPS First Class is requested.

About replacing the rectifier in any boatanchor: you can do it, but the B+
is going to go way up, so you need to make some measurements 1st. Measure
the HV, then the current. Replace whatever rectifier, then again measure the
B+. That will give you a value for an appropriate dropping resistor-both
ohms and power. Add the resistor, measure until you have the resistor value
right, and you are back to operation with the original voltage level.

** And at no heat savings other than the tube filament and at the same time
the transformer is running harder due to the hard switching of the SS.
You wind up with a net gain of zero in most of those swaps.

Obviously, this is much more reasonable when there isn't a lot of current,
and when that current is steady-such as in a T/R Switch.

Other points about T/R Switches:
1) Fusing. I always add a subminiature 250VAC-rated TO-5
transistor-sized fuse (typically a 1 or 2A SB) internally. Fuses are
primarily disaster prevention devices, and for something like a T/R Switch 1
or 2A SB is fine. The EFJ 250-39 uses the EFJ fused-plug with a 2-wire cord.
By adding a small internal fuse, you can use a 3-wire cord which I am much
more comfortable with, and without violating my Rule 1-for-Boatanchors: I
don't make no new holes.
2) Rectifiers. I (pardon the heresy here on EFJ) really like the
broadband B&W 380B and the B&W 381-which adds bandswitching, and hence some
selectivity. The latter T/R SW is somewhat hard to find.

** I use a pair of Johnsons and a pair of B&W 381-C's, the B&W has a better
SNR on 10 and 15M but only with a fairly noisy radio to start with. None of
the T/R switches will help already hot performers and just raise the noise
plus signal.

The B&Ws use a
selenium rectifier and a 2 wire cord with no fusing (there MAY be an
internal fusible link (one individual wire from the stranded 2 wire cord
wire.) Replacing any selenium rectifier is always a good idea since the
fumes from a burned-out selenium rectifier are a definitely poisonous gas.

** First Ive ever heard the selenium fumes are toxic or a poisonous gas.
They will certainly clear a room/house with the stink but AFIK they havent
killed anyone and millions were used in consumer electronics (-; including
Johnsons.

Carl
KM1H
-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net] On Behalf Of Charles Ring

I have used the 250-39 Johnson T/R switch since 1967. I've had 3 of them and
the power transformer on two of those have failed. Since returning to the
air a year ago I've been using one with my Globe King 500A and Drake R-4C.
It works, but unlike long ago, I can't get full QSK. The receiver is being
badly desensed. I don't remember if it did better then with the R-4C or if
that was only with the 2-C and the R-4B, both of which are waiting for
repairs. I am interested in changing to either a fast relay or a PIN diode
switch. I seldom do faster than 25WPM.

73 de W3NU

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